Not applicable.
This invention relates to pressure swing adsorption (PSA) systems. More particularly, it relates to the use of a PSA system in the separation of a gas mixture containing oxygen and nitrogen, and the removal of feed impurities, such as water and/or carbon dioxide, by using lithium-containing FAU adsorbents.
PSA systems are particularly suitable for a wide variety of important commercial applications. For example, high purity oxygen is used in various industries, such as chemical processing, steel mills, paper mills, and lead and gas production operations.
In a PSA process, a feed gas mixture, such as air, containing a more readily adsorbable component and a less readily adsorbable component, e.g., the nitrogen and oxygen components of air, is passed through the feed end of an adsorbent bed capable of selectively adsorbing the more readily adsorbable component at an upper adsorption pressure. The less readily adsorbable component passes through the bed and is recovered from the discharge end of the bed. Thereafter, the bed is depressurized to a lower desorption pressure for desorption of the more readily adsorbable component, and its removal from the bed prior to the introduction of additional quantities of the feed gas mixture for repressurization and adsorption as cyclic adsorption-desorption-repressurization operations are continued in the bed. Such PSA processing is commonly, but not exclusively, carried out in multi-bed systems, with each bed employing the PSA processing sequence on a cyclic basis interrelated to the carrying out of such processing sequence in the other beds of the adsorption system.
In PSA systems for the recovery of high-purity oxygen product as the less readily adsorbable component of air, each adsorbent bed will commonly contain an adsorbent material capable of selectively adsorbing nitrogen as the more readily adsorbable component, with the selectively adsorbed nitrogen being subsequently desorbed from the bed upon reduction of the pressure of the bed from the upper adsorption pressure level to the lower desorption pressure level. When the lower pressure level is below atmospheric pressure, it is generally referred to as Vacuum Swing Adsorption (VSA). However, for purposes of simplicity, the term xe2x80x9cPSAxe2x80x9d shall be used hereinafter to denote both PSA and VSA systems unless noted otherwise.
In addition to nitrogen and oxygen, a feed gas mixture may contain impurities, such as water and carbon dioxide. Conventional wisdom teaches that it is necessary to remove water and carbon dioxide, so as to avoid poisoning the nitrogen adsorbing capacity of high-capacity main-stage adsorbents. This removal can be accomplished through the use of either a separate pretreatment material or a separate pretreatment stage.
For instance, Canadian Patent Application No. 2,234,924 to Ackley discloses the removal of feed stream contaminants, typically water and carbon dioxide, in a pretreatment stage at the feed end of the adsorbent bed, by use of material selected from the group consisting of zeolites, activated alumina, activated carbon and silica gel. Lithium-exchanged zeolites are taught to be useful in the main stage of the disclosed process.
Canadian Patent Application No. 2,182,641 discloses a PSA process using two layers of adsorbent materials comprising a first adsorbent layer of NaX and a second adsorbent layer of LiX. It also discloses that the gas stream can be dried before passage through the zeolite packing, by means of a drying layer of silica gel.
U.S. Pat. No. 5,810,909 discloses the use of a pretreatment zone containing, e.g., alumina, to remove water and carbon dioxide before bulk separation through multiple adsorbent layers that can include lithium-exchanged zeolites.
Likewise, Rege et al., xe2x80x9cLimits for Air Separation by Adsorption with LiX Zeolitexe2x80x9d Ind. Eng. Chem. Res. (1997), vol 36, pp. 5358-5365, teaches the use of a pretreatment bed to remove water and carbon dioxide from the feed gas before it enters the main LiX bed.
U.S. Pat. No. 3,636,679 discloses an apparatus in which CaA is used as the only adsorbent and the inventors explicitly state that the air is fed without pretreatment to remove water or CO2. However, CaA""s air separation performance is inferior to those of lithium-exchanged FAUs.
U.S. Pat. No. 5,133,784 discloses an apparatus in which flow is radial in a bed composed of co-annular cylinders. The apparatus is said to be suitable for separating at least one component, such as oxygen, from a gaseous mixture. Alumina is the only adsorbent mentioned.
U.S. Pat. No. 5,203,887 discloses replacing a portion of an adsorbent bed of the lithium-exchanged zeolite type with an adsorbent of another type. This is achieved by utilizing an adsorption zone including two adsorbent beds arranged in series. The first bed comprises a zeolite X exchanged to at least 80% with lithium, and the second bed comprises an unexchanged conventional zeolite X, such as NaX. The precise nature of the feed gas is not described, and there is no suggestion in this patent that feed gases containing carbon dioxide and/or water can be fed to the lithium-exchanged zeolite without pretreatment to remove carbon dioxide and water.
U.S. Pat. No. 5,658,370 discloses a process for the separation of nitrogen from a gaseous mixture, such as air, by selective adsorption of nitrogen on an adsorbent mass, using a rotating radial flow bed geometry, wherein at least 50% of the adsorbent mass consists of at least one lithium-exchanged zeolite with an exchange level ranging from 50% to 95%. However, this patent is silent on the issue of the removal of water and carbon dioxide.
Avgul et al., xe2x80x9cHeats of Adsorption on X-Type Zeolites Containing Different Alkali Metal Cationsxe2x80x9d, Molecular Sieve Zeolitesxe2x80x94II, Advances in Chemistry Series 102, 1971, pp. 184-192, compares the heat of adsorption of water in LiX to that of NaX. The authors speculate that this initial water adsorption probably occurs on the SIII cation sites. Li, Na, and K-exchanged X all show initially high heats of adsorption. Li and NaX are very similar3 after water adsorption progresses to the SII, and SIxe2x80x2, cation sites.
Information on CO2 adsorption is provided by Vasil""eva and Khvoshchev, xe2x80x9cHeats of Adsorption of CO2 and NH3 on Synthetic Zeolites of Different Structural Types. Communication 3. CO2 Adsorption on Li, Na, and K forms of X and Y Zeolites.xe2x80x9d, Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp1942-1947, September 1984 (translation by I. V. Grebenshchikov, Institute of Silicate Chemistry). In FIG. 4 of this article, the CO2 heats of adsorption for KX, NaX, and LiX are plotted as a function of CO2 loading. For the first 20 or so CO2 molecules per unit cell, the heat of adsorption is highest for LiX. However, beyond that loading, the heat of adsorption decreases for LiX and actually becomes much lower than for NaX or KX. This behavior is similar to that reported by Avgul et al. for the water heat of adsorption. Thus, after a certain initial loading, the LiX actually has equal or lower affinity for water and carbon dioxide than the common pretreatment adsorbent, NaX.
It would be desirable to provide a process for isolating oxygen from a feed gas containing oxygen, nitrogen, and at least one of water and carbon dioxide, wherein at least one adsorbent used to separate oxygen from nitrogen can also remove water and carbon dioxide from the feed gas without being entirely poisoned thereby.
All references cited herein are incorporated herein by reference in their entireties.
Accordingly, the invention provides a process for separating a feed gas into at least one product gas, comprising: (a) providing a gas separation apparatus comprising at least one adsorption layer comprising a lithium-exchanged FAU having water desorption characteristics, defined by drying curves, similar to those for the corresponding fully sodium-exchanged FAU, a heat of adsorption for carbon dioxide equal to or lower than that for the corresponding fully sodium-exchanged FAU at high loadings of carbon dioxide, and onto which said adsorption layer water and/or carbon dioxide adsorb, (b) feeding into the gas separation apparatus a feed gas including nitrogen, oxygen, and at least one of carbon dioxide and water; and (c) collecting from a product end of the gas separation apparatus at least one product gas comprising oxygen.